In a compressor of a turbomachine, and in particular of a turbojet, one or more sets of stator vanes may be of variable pitch, i.e. the angle of attack of such vanes can be varied as a function of flight conditions under the control of a servo-control system that moves a band connected via links to the pivots of the vanes in a given set, causing the band to turn and to move axially as a function of predefined parameters. Such variations in vane pitch, associated with vibration of the casing, leads to wear of contacting parts. It has been found that wear increases particularly when angular offsets are small and the frequency of vibration is in the vicinity of 160 hertz (Hz). The most harmful element is the peening pressure on the pivots, which is harmful to the strength of the parts.
For a variable-pitch system in a turbomachine, there is a special constraint constituted by the need to operate in a non-lubricated medium. Only dry lubrication can be envisaged for improving contact conditions and for increasing the lifetime of the parts.
To satisfy those needs, in French patent application No. FR 03/00435, filed on Jan. 16, 2003 and published on Jul. 23, 2004 under the No. FR 2 850 138, the Applicant proposes a smooth bearing comprising a ceramic bushing inside which there is placed a pivot based on iron, nickel, or cobalt, or a metal pivot carrying a shrunk-on insert based on iron, nickel, or cobalt, with the oxides of iron, nickel, or cobalt that are created during continuous contact between the vane pivot and the smooth bearing, acting as a lubricant.
One of the factors limiting the use of ceramics as material for making bushings is their brittleness and their low toughness. It thus happens that bushings break, either while being mounted on the casing, or else in operation because of the different coefficients of expansion between the metal of the casing and the ceramic and because of the shocks received by the systems.